The present invention relates to a novel fluorine-containing meltable resin composition. In particular, the present invention relates to a fluorine-containing meltable resin composition comprising a fluorine-containing resin to which a specific needle-shape material is compounded, having good sliding and sealing properties and being able to be injection molded, which composition is suitable for dynamic sealing materials such as seal rings for automatic transmissions of automobiles, seal rings for hydraulic devices for braking, seals for power steerings, seal rings for CVTs, piston rings for shock absorbers, chip seals for compressors of air conditioners, valve seals, tubes for push-pull cables, gaskets, and bearings.
Hitherto, tetrafluoroethylene polymer compositions containing inorganic or organic fillers for improving dimensional stability, creep resistance, wear resistance and the like are known as materials used for producing sliding parts for automobiles or industrial equipments. However, the dimensional stability and creep resistance are not satisfactorily improved by the fillers. Furthermore, these compositions have a drawback that they cannot be injection molded, and therefore, they have low productivity and are not preferable in view of production costs.
For example, in the processing of pistons for shock absorbers, a peripheral surface is shaped in a special form, and a layer of polytetrafluoroethylene (PTFE) is wrapped around the peripheral surface by press molding and then fusion bonded to prevent the deterioration of durability due to twisting of the PTFE resin layer which is caused by repeated reciprocal movement because of low adhesion between PTFE and the piston.
Some compositions have been proposed, which comprise injection moldable heat resistant resins and various solid lubricants or for reinforcing materials for imparting sliding properties to such the resins or reinforcing them. However, such the compositions have inferior sliding properties to the tetrafluoroethylene polymer compositions, and furthermore, they are too stiff to achieve sufficient sealing properties in applications where the sealing properties are also required.
With the decrease of weights of various equipments in these years, soft materials such as aluminum are increasingly used. When such the soft materials are used as opposite substrates, the above conventional resin materials damage the substrates during sliding movement. Furthermore, no compositions satisfy the sliding properties and other properties when they are used in applications which require the high sliding properties, for example, the use at high speed under high load.
JP-A-6-200280 discloses a multilayer sliding part comprising a part substrate and a lubricating composition which comprises a fluororesin and zinc oxide whiskers and is impregnated in and coated on the substrate. However, this composition uses PTFE having no flowability as a matrix, and cannot be injection molded. In addition, the advantages of fillers having tetrapod-form three dimensional shapes represented by the zinc oxide whiskers cannot be attained when they are added to a material which is expected to be used for melt molding.
JP-A-60-120798 discloses a sliding member comprising a fluororesin and silicon carbide whiskers. The addition of filler having high hardness such as the silicon carbide whiskers can improve the wear resistance of the resin composition, but may cause a problem that the composition wears the substrate over which the sliding member is slid, due to the excessively increased attacking properties of the composition.
JP-A-5-117475 discloses a resin composition comprising a fluorine-containing meltable resin and carbon whiskers (carbon fibers). This composition has several excellent properties and also good sliding properties because of the inherent sliding properties of the carbon whiskers having low hardness. However, the sliding properties may be still insufficient for the applications requiring high sliding properties such as the use at high speed under high load.
In general, the fluorine-containing meltable resins are said to have lower flowability and less injection moldability than other thermoplastic resins. Meltable resin compositions comprising the fluorine-containing meltable resin and common fillers tend to form blisters or flow marks on the surface when molded articles have a thin wall, that is, they have low moldability of thin-wall articles.
The present invention intends to solve the drawbacks of the conventional fluororesin compositions, in particular, the fluorine-containing meltable resin compositions and provide a fluorine-containing meltable resin composition which is preferable as a material for sliding parts for automatic transmissions of automobiles, sliding parts for pistons of shock absorbers, and various types of seals.
According to the first aspect, the present invention provides an injection moldable fluorine-containing meltable resin composition comprising 70 to 98 wt. % of a fluorine-containing meltable resin, and 2 to 30 wt. % of a needle-shape material having an average diameter of 3 xcexcm or less, an average aspect ratio of between 10 and 100, and Mohs hardness of between 5 and 8.
According to the second aspect, the present invention provide a dynamic sealing material such as a sealing ring for automatic transmissions of automobiles, consisting of a molded article of the fluorine-containing meltable resin composition according to the present invention, or a shock absorber comprising a piston and a piston ring which is placed around the peripheral surface of the piston wherein the piston ring consists of a molded article of the fluorine-containing meltable resin composition according to the present invention.
According to the third aspect, the present invention provide a method for producing the above dynamic sealing material.